The present invention relates to a tire more particularly to a tread groove structure being capable of reducing demolding defects caused by a single-split mold.
In comparison with a segment mold which comprises circumferentially divided, radially movable sectors, a single-split mold or two-piece mold having a split face on or near the tire equator is knows as low cost because the mold itself and incidental equipment, controller and the like are simple.
When such a single-split mold is opened to take out the vulcanized tire therefrom, as the two mold pieces are moved outwards in the tire axial direction, grooved parts in the tread portion of the tire are liable to be damaged by protrusions which are provided on the inside of the mold in order to make tread grooves. Such demolding defects are more liable to occur as the inclination angle of the groove becomes smaller with respect to the tire circumferential direction and in the tread central portion than the tread shoulder portion.
On the other hand, ordinary pneumatic tires are provided in the tread portion with a circumferential groove and/or a less-inclined oblique groove. Accordingly, it is difficult to manufacture such tires using a single-split mold although it can reduce the tire production cost.
It is therefore, an object of the present invention to provide a pneumatic tire which can be made without demolding defects using a single-split mold even if a circumferential groove and/or less-inclined oblique grooves are disposed in the tread central portion.
According to the present invention, a tire comprises
a tread portion in which a mold parting line by a single-split mold runs on or near the tire equator defining a right-hand part of the tread portion on one side of the mold parting line and a left-hand part on the other side,
the tread portion provided in at least a central part with tread grooves each having a right-hand groove wall and a left-hand groove wall, wherein
in the right-hand part, the right-hand groove walls are at least partially provided with an inclined part extending from the tread face towards the groove bottom while inclining to the left, and the left-hand groove walls are inclined less than the inclined part of the respective opposite right-hand groove walls,
in the left-hand part, the left-hand groove walls are provided with an inclined part extending from the tread face towards the groove bottom while inclining to the right, and the right-hand groove walls are inclined less than the inclined part of the respective opposite left-hand groove walls, and
the inclined parts are
a first inclined part extending from the tread face to the vicinity of the groove bottom, while inclining at an first inclination angle of from 15 to 45 degrees, and/or
a second inclined part inclined at a second inclination angle of from 30 to 60 degrees and having a width of from 0.3 to 2 mm when measured in parallel to the tread face and accompanied by a recess which is narrower and shallower than the tread groove and extends along the second inclined part.